Mass spectrometry generally refers to the direct measurement of the value of a particle's mass or an implicit determination of the value of the particle's mass by measurement of other physical quantities using spectral data. Mass spectrometry often involves determining the mass-to-charge ratio of an ionized molecule or component. When the charge of the ionized particle is known, the mass value of the particle can be determined from a spectrum of mass values.
Systems for performing mass spectrometry are known as mass spectrometers. Mass spectrometer systems generally include an ion source, a mass filter or separator, and a detector. For example, a sample of molecules or components can be ionized by electron impact in the ion source to create ions. Ions having different mass values are separated by the mass analyzer into a mass distribution or spectrum, for example, by application of electrical or magnetic fields to the ions. The detector collects the ions, and the mass distribution may be viewed and/or recorded. The relative abundance of mass values in the spectrum is used to determine the composition of the sample and the mass values or identities of molecules or components of the sample.
Many different types of mass spectrometer exist, including a category referred to as ion-molecule reaction mass spectrometers (IMR-MS). Within this category, several technologies exist including proton transfer reaction mass spectrometry (PTR-MS) and selected ion flow tube mass spectrometry (SIFT-MS). Such categories generally refer to the method by which ions are generated. For example, proton transfer reaction mass spectrometers include an ion source that generates reagent ions, typically hydronium ions (H3O+), to transfer charge to sample components, e.g., by proton transfer. In selected ion flow tube mass spectrometers, a carrier gas transports filtered ions along a flow tube. In proton transfer reaction mass spectrometers sold by Ionicon Analytik GmbH of Innsbruck, Austria, a hollow cathode tube is used as an ion source to produce reagent ions by applying a DC plasma discharge to a stream of water vapor.
Some mass spectrometry systems are classified by the type of mass analyzer used. For example, some mass spectrometry systems are based on “tandem techniques” where another analytical technology is used in combination with mass spectrometry equipment. An example is gas chromatography mass spectrometry (GC-MS) where a gas chromatography column is used to separate components of a sample prior to analysis with a mass spectrometer.
Mass spectrometry can be used to determine the quantities of volatile organic compounds (VOCs) in a sample. Measurement of VOCs has become important because the presence of VOCs, even in trace quantities, can serve as an important diagnostic indicator in many different applications and may affect human health. For example, when the concentration of VOCs rises above a certain level, detrimental health effects can occur in humans such as respiratory conditions. Moreover, the type and quantities of VOCs in a particular sample can be indicative of the presence of explosives, harmful chemical agents, combustion products, disease agents, decay or contamination, arson accelerants, or drugs of abuse. Additionally, monitoring the presence and quantity of VOCs is useful in industrial processing, such as biochemical or pharmaceutical manufacturing processes.
Several drawbacks are inherent with existing mass spectrometry systems both generally and as applied to detection of VOCs. For example, mass spectrometry systems employing gas chromatography are not suitable for continuous, real-time monitoring of a fluid sample due to the relatively slow analysis of a sample. Moreover, previous mass spectrometry systems often require collection of a sample from the field prior to analysis of the sample in a lab-based environment, rather than in situ analysis. Previous mass spectrometry systems are relatively insensitive to lower-concentration components in a sample, for example, because ion sources do not produce a sufficient quantity of ions to generate an identifiable mass spectrum for lower-concentration constituents. The mass spectrum for lower-concentration constituents in such systems is often indistinguishable from noise due to dynamic range limitations or overwhelmed by peak interference from higher-concentration components or noise generated by electronic or mechanical equipment. Mass spectrometry systems with suitable levels of sensitivity can facilitate detection of the existence of VOCs, but may be subject to interference from other compounds present and therefore unable to positively identify a particular compound or species.